Years of schooling were an imperfect proxy for cognitive ability in a study of methamphetamine dependence. Simply adjusting for education can therefore change an analysis without necessarily removing the influence researchers intend to control.
Dean and colleagues evaluated 36 methamphetamine-dependent participants and 42 healthy comparisons with cognitive testing and self-report measures. They used a cognitive battery to assess general cognition and vocabulary as a proxy for academic learning. This was observational human primary research, not a drug-administration experiment or an intervention to increase schooling.
Earlier onset of methamphetamine use was associated with fewer years of education. Relative to a model developed in healthy comparisons, dependent participants had less education than predicted from demographics and cognitive performance. Larger predicted-versus-observed education discrepancies were associated with earlier use onset. Years completed before use began predicted vocabulary better than total years of education.
If school interruption is partly related to use, education may be both a background characteristic and part of the pathway under examination. Treating it only as a measure of premorbid intelligence can obscure that distinction. Conversely, omitting education without justification can leave genuine confounding. The paper does not show that one adjustment strategy is correct for every study.
The sample was small, the design was cross-sectional and retrospective history cannot establish exact causal direction. Vocabulary was itself a proxy, not a complete measurement of educational quality. These results do not prove that methamphetamine caused a specific person’s schooling or cognitive difficulty. No concentration or specimen-based impairment threshold was tested. Forensic review should examine prior records, learning opportunity, onset chronology and direct testing. Educational attainment can supply useful context, but it should not replace individualized measurement or be treated as an exact estimate of original ability.
For a separate example of the limits of use-history comparisons, see our current-user driving study. The design and measured outcome determine which conclusions can be carried into an individual case.
Original investigation: doi:10.1016/j.drugalcdep.2011.11.019.